The continuous 7-hour intravenous dexamethasone suppression test in the differential diagnosis of ACTH-dependent Gushing's syndrome

OBJECTIVE A recent report showing disappointingly low sensitivity and speci ficity for the oral high dose dexamethasone test in the differential diagno sis of Gushing's syndrome prompted us to re-evaluate the results obtained i n our centre using the continuous 7-hour intravenous dexamethasone suppress ion test for this purpose. PATIENTS 105 patients with AGTH-dependent Gushing's syndrome were included in this study; 78 with Gushing's disease, 8 with ectopic ACTH-secreting tum ours and 19 were classified as 'of unknown aetiology'. RESULTS In 74/78 (94.9%) of patients with Gushing's disease and in 3/8 (37. 5%) patients with the ectopic ACTH syndrome, a plasma cortisol decrease >19 0 nmol/l at 7 h as compared to baseline values was achieved in the continuo us 7-hour intravenous dexamethasone suppression test. Using a plasma cortis ol decrease >190 nmol/l at 7 h as compared to baseline values as the cut-of f value, the sensitivity and specificity of the continuous 7-hour intraveno us dexamethasone suppression test for the diagnosis of Gushing's disease in patients with AGTH-dependent Gushing's syndrome were 94.9% and 62.5%, resp ectively. CONCLUSIONS In patients with AGTH-dependent Gushing's syndrome with a plasm a cortisol decrease >190 nmol/l at 7 h in the continuous 7-hour intravenous dexamethasone suppression test, additional localizing investigations such as bilateral simultaneous inferior petrosal sinus sampling and/or pentetreo tide scintigraphy should be performed when no clearly discernible pituitary adenoma is observed on MRI studies. Patients with ACTH-dependent Gushing's syndrome with a plasma cortisol decrease <190 nmol/l at 7 h in the continu ous 7-hour intravenous dexamethasone suppression test should also undergo b ilateral simultaneous inferior petrosal sinus sampling and/or pentetreotide scintigraphy to demonstrate the presence of a nonpituitary source of ACTH overproduction.